Accordingly, one object of the present invention is to provide a solder ball fabricating process for directly forming a solder ball on the bonding pad of a wafer in a wafer level chip scale package (WLCSP). The solder ball fabricating process not only increases production rate, but the size and height of the solder ball is also much easier to control within the desired range. In addition, constituents inside a lead-free solder ball are easier to control when the invention is applied to form lead-free solder balls.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a solder ball fabricating process for attaching at least one solder ball to a wafer. The wafer has an active surface, a passivation layer and at least one bonding pad. The passivation layer and the bonding pad are formed on the active surface of the wafer such that the passivation layer exposes the bonding pad. The wafer further includes a stress buffer layer and at least one under-ball-metallurgy layer. The under-ball-metallurgy layer is formed over the bonding pad. The stress buffer layer is formed over the passivation layer but also exposes the under-ball-metallurgy layer. First, a patterned first solder mask layer is formed over the stress buffer layer. The first solder mask has at least one first opening that exposes the under-ball-metallurgy layer. A first solder material is deposited into the first opening and then a first reflow process is carried out so that the first solder material inside the first opening is turned into a pre-solder body. Thereafter, a patterned second solder mask layer is formed over the first solder mask layer. The second solder mask layer has at least a second opening located above the first opening and exposing the pre-solder body. The second opening also has a diameter greater than the first opening. A second solder material is deposited into the second opening and then a second reflow process is carried out so that the second solder material inside the second opening and the pre-solder body melt together to form a solder ball above the under-ball-metallurgy layer. Finally, the first solder mask layer and the second solder mask layer are removed.
The solder ball fabricating process according to this invention includes forming a solder mask layer over the wafer and patterning the solder mask layer to form an opening. The opening exposes the under-ball-metallurgy layer above the bonding pad of the wafer. Thereafter, a solder material is deposited into the opening so that the solder material stacks on top of the under-ball-metallurgy layer. A reflow process is conducted next to melt the solder material into a pre-solder body. The aforementioned steps are repeated once so that various solder materials are melted into a solder ball above the bonding pad. Note that diameter of the opening in each solder mask layer may not be the same. Hence, a staircase-like or an inverted frustum-cone-like cavity structure is formed. This type of cavity structure facilitates the deposition of solder material into the opening. Therefore, solder ball having greater size and height are formed over the under-ball-metallurgy layer after the reflow of various solder materials is completed.
Similarly, the solder ball fabricating process according to this invention can be applied to fabricate lead-free solder balls. Since the constituents of a lead-free solder ball include metallic substances or alloys combined in various ratios, the lead-free solder balls are formed by sequentially stacking various types of solder materials over the bonding pads of the wafer and then melting the solder materials together. By adjusting the thickness of each solder mask layer and size of each opening, volume of solder material deposited into each opening can be precisely adjusted so that the ultimately formed lead-free solder ball has the desired height and contains all the necessary constituents mixed in the desired ratio.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.